Download the Custom HomeMenu Manager (CHMM) homebrew so you can install the themes. In the pack you have all versions of the app, for Ninjhax and Gateway.

After downloading the theme pack, create a folder on the SD named "Themes" and extract the themes you want there.

After that and booting your console, you can load CHMM and you will see a list of themes you extracted before. Select the theme you want and press A, the app will freeze for a couple seconds, after that press Start and you will be back to your Home menu, but the theme isn't active yet. You need to press the Power button once and then Home to go back, the theme will be loaded.

To change the theme again, just do the same steps. In case you want your own preview image of the theme, you can take an screenshot using the console screenshot feature, copying the image to the theme folder and renaming it to "preview.png".